Microfluidic devices are well known for generating droplets. The droplets are aqueous compartments which are formed by combining aqueous and oil flows in a microfluidic device. Either the aqueous or the oil flow will form the droplets while the other flow forms the continuous phase. The liquids which are used in the microfluidic device are typically stored away from the microfluidic chip and are typically connected to the chip via a tube. The liquid is pushed from the reservoir to the chip along the tube either using gas pressure, or using a syringe. However, connections between a tube and a chip with microchannels can be problematic. This is because connections typically have changes in the flow path cross-section and un-swept volumes where particles can get trapped. It is therefore preferable to locate the reservoirs in the microfluidic device itself. This eliminates the above-mentioned connections and also allows the channel geometries between the reservoir and microfluidic channel to be optimised to reduce sedimentation which might otherwise occur, for example, in cell and bead suspensions.
Another advantage of integrating the fluid reservoirs into the microfluidic device is that all of the fluids are contained in a single component which can be disposed easily. This is important for many biological applications where avoiding cross-contamination between samples is a key requirement.
One way to achieve such an integrated microfluidic device is to provide a block comprising at least one reservoir and a base layer beneath the block to form the bottom of the reservoir. Microchannels can then be formed in the bottom of the block and/or the top of the base layer of the outer face between the block and base layer, the channels being in fluid communication with the reservoirs.
The liquid can then flow under gravity from the reservoir into the microfluidic channel below.
However, this can give rise to a problem as explained with reference to FIGS. 1A to 1D. These figures are all schematic plan views of the microfluidic channel 1. The microfluidic channel comprises two aqueous channels 2 for aqueous solution 8 which converge at a junction 3. A first aqueous solution typically containing cells will be fed along one channel while RNA capture beads with a lysis buffer be fed along the second channels. A pair of oil channels 4 feed oil to a second junction 5 downstream of the first junction 5 and an emulsion is formed at outlet channel 6 as best shown in FIG. 1C which shows oil 7 as the continuous phase with the droplets of the aqueous solution 8. The reservoirs which supply the aqueous solution 8 and oil 7 are provided in a block which is above the plane of the paper shown in FIGS. 1A to 1D such that these flow into the channels under gravity.
FIG. 1A shows the initial state of the microfluidic channel before any fluids have flowed into the channel 1. In this state, all of the channels are filled with air. The channel is ideally primed as shown in FIG. 1B with the aqueous solutions 8 and oil 7 having displaced most of the air out through the outlet channel 6.
If, however, the oil 7 reaches the junction 5 for the aqueous solutions 8, oil will typically flow into the aqueous channels 2 as shown in FIG. 1D.
If the aqueous solutions 8 have been dispensed into the channels 2 then the oil 7 will not flow very far into the aqueous channels 2 as the air in these channels will be trapped and will get compressed and resist the flow of the oil 7. When the flow of the aqueous solutions 8 is initiated then the air and the aqueous solutions 8 will push most of this oil out, however the oil can get stuck to the inside surfaces of these channels. If oil droplets do get stuck then they will affect the hydraulic resistance of the channels which in turn will affect the flow rate of the aqueous phase and affect the quality of the droplets produced.
If the aqueous solutions 8 have not been dispensed into the aqueous channels 2 then the oil 7 can potentially flow all the way up the aqueous channels 2 and into the aqueous reservoirs. This would cause significant problems when trying to run an aqueous sample. The only way to clear the oil out of the microfluidic channel 1 is to pump the aqueous solutions 8 through the channel 1. While this ‘purging’ process is occurring the quality of any droplets produced is likely to be poor. This results in loss of sample and potential corruption of the emulsion that is collected in the output reservoir.
One way of addressing this problem is to provide a capillary valve as disclosed, for example, in “Droplet on demand system utilizing a computer controlled microvalve integrated into a stiff polymeric microfluidic device; Krzysztof Churski, Jacek Michalski and Piotr Garstecki; Received Jul. 24, 2009, Accepted Oct. 28, 2009; First published as an Advance Article on the web Dec. 1, 2009; DOI: 10.1039/b915155a”. This is a channel with a series of wider regions which are sufficiently wide that the capillary force can no longer draw the liquid through this region. Whilst this works with aqueous solutions, it is not effective for oil based liquids.